As FIG. 59 illustrates, a conventional data base is configured to store data in each hard disk drive (HDD).
Table A in FIG. 59 consists of four subtables A0 through A3, each of which holds a number of records stored in hard disk drives correspondingly from HDD0 to HDD3. Coupled to a hard disk drive, slave processors 0 though 3 (designated as slave 0 to slave 3 in the drawings), can access subtables held in the hard disk drives.
A processor built in each slave processor enables the slave processor to carry out processing locally.
A master processor (referred to as the master in the drawings) can access Table A by issuing a request to any of the slave processors. The master processor, equipped with its own built-in processor, retrieves data upon request from a host computer and sends the retrieved data to the host computer.
FIG. 60 illustrates a method of retrieving data for the conventional high-speed data access machine. Hard disk drives contain logical and physical address conversion tables, T0 through T3 , in addition to subtables A0 to A3. Hard disk drives 0 through 3 store the content of subtables A0 to A3.
The address conversion table, T0 in this example, has logical addresses that indicate the addresses of records in tables and the physical addresses that indicate the addresses on a hard disk drive.
A slave processor reads the logical and physical address conversion table from an HDD and have it reside in its own memory. Then it converts the record address or the logical address in the table requested from the master processor, into an address on a disk, namely physical address. The physical address is used to access an HDD to retrieve data, which is output to the master processor from a slave processor.
After the requested data is retrieved from the slave processor, the master processor performs necessary processing and outputs its result to the host computer.
FIG. 61 shows a conventional distributed data base system in which a client, linked to a local area network, accesses data bases 0 to 3. Data bases 0 through 3 store the content of table A which is divided into four subtables, subtables A0 to A3. Problems to be solved by this Invention
The biggest problem posed by the conventional distributed data base system is that if a malfunction or a glitch occurs to any of the hard disk drives coupled to the slave processors, the whole system goes down, thus disabling data retrieval.
Moreover, system recovery is time consuming in the event of a malfunction, because data must be reloaded from the host computer into a replaced new HDD.
In addition, rotational delay of an HDD and the seek time taken in accessing data add up to a sizable amount of overhead.
On top of that, a conventional distributed data base system is equipped with only one master processor. It limits the number of slave processors to be coupled in light of the load capacity of the master processor to bear. This is disadvantageous for the increase in the number of slave processors is highly desirable to upgrade parallel processing, thereby achieving more effective and powerful data manipulation. However, in reality, with one master processor, the conventional data access apparatus is unable to accommodate high-speed data manipulation, most notably join processing.
With respect to the distributed data base system presented in FIG. 61, the client would have to access several data bases individually. This increases the load on the part of the client when the client needs to collect data from a plurality of subtables or to perform a joint processing for data from multiple tables.
Furthermore, the function of allocating data to several data bases is entirely rested upon the client, adding another workload of data maintenance to the client.
Accordingly, it is an object of the present invention to achieve high-speed data access by solving the above-mentioned problems. It aims at greater data access capability and higher reliability of the data access apparatus.
Another object of the present invention is to facilitate data recovery in time of system failure in a memory of a data access apparatus.
Another object of this invention is to increase data reliability by creating redundant data In addition to the data distributed in a plurality of hard disk drives.
Also, it is another object of the present invention to upgrade the performance without increasing the number of slave processors.
It is also another object of this invention to reinforce parallel processing capability by adding the number of slave processors without causing any additional load on the master processor.
Another prime object of this invention is to relieve the client from the extra workload of data allocation so as to enable the client to retrieve and maintain distributed data freely.